IMA 2021-033 = sergeysmirnovite

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Marco E. Ciriotti
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IMA 2021-033 = sergeysmirnovite

Messaggio da Marco E. Ciriotti » mer 10 ago, 2022 15:24

Risoluzione strutturale della nuova specie sergeysmirnovite.

Referenza:
▪ Krivovichev, S.V., Panikorovskii, T.L., Yakovenchuk, V.N. (2022): The Crystal Structure of Sergeysmirnovite, MgZn2(PO4)2·4H2O, and Complexity of the Hopeite Group and Related Structures. Crystals, 12, 1120.

Abstract:
The crystal structure of sergeysmirnovite, MgZn2(PO4)2·4H2O (orthorhombic, Pnma, a = 10.6286(4), b = 18.3700(6), c = 5.02060(15) Å, V = 980.26(6) Å3, Z = 4), a new member of the hopeite group of minerals, was determined and refined to R1 = 0.030 using crystals from the Këster mineral deposit in Sakha-Yakutia, Russia. Similar to other members of the hopeite group, the crystal structure of sergeysmirnovite is based upon [Zn(PO4)]– layers interlinked via interstitial [MO2(H2O)4]2– octahedra, where M = Mg2+. The layers are parallel to the (010) plane. Within the layer, the ZnO4 tetrahedra share common corners to form chains running along [001]. Sergeysmirnovite is a dimorph of reaphookhillite, a mineral from the Reaphook Hill zinc deposit in South Australia. The relations between sergeysmirnovite and reaphookhillite are the same as those between hopeite and parahopeite. Topological and structural complexity analysis using information theory shows that the hopeite (sergeysmirnovite) structure type is more complex, both structurally and topologically, than the parahopeite (reaphookhillite) structure type. Such complexity relations contradict the general observation that more complex polymorphs possess higher physical density and higher stability, since parahopeite is denser than hopeite. It could be hypothesized that hopeite is metastable under ambient conditions and separated from parahopeite by a structural and topological reconstruction that requires an essential energy barrier that is difficult to overcome.

NdR:
Hopeite group Pnma
Hopeite ZnZn2(PO4)2·4H2O
Nizamoffite Mn2+Zn2(PO4)2⋅4H2O
Rollandite CuCu2(AsO4)2·4H2O
Sergeysmirnovite MgZn2(PO4)2⋅4H2O
Wattevilleite CaNa2(SO4)2·4H2O

Parahopeite group P1
Davidlloydite ZnZn2(AsO4)2·4H2O
Parahopeite ZnZn2(PO4)2·4H2O
Reaphookhillite MgZn2(PO4)2·4H2O


Hopeite related structures
Phosphophyllite Fe2+Zn2(PO4)2·4H2O mon P21/c
Steinmetzite Fe3+Zn2(PO4)2(OH)·3H2O tric P1
Stergiouite CaZn2(AsO4)2·4H2O mon Pc


C'è qualcosa che non quadra alla perfezione nella struttura proposta...
Marco E. Ciriotti

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